1. Field of the Invention
The present invention relates to a vented compartment inerting device for quickly and selectively rendering a vented compartment nonexplosive and, more particularly, to a method and apparatus for rapid and efficient inerting of a vented compartment.
2. Description of the Related Art
According to the Federal Aviation Administration, at least ten airplane loss events between 1959 and 1998 have involved fuel tank explosions. There is a thus a need in the aviation industry for eliminating, or reducing the risk due to explosive fuel-air mixtures in fuel tanks of airplanes. The U.S. Federal Aviation Administration has studied various potential solutions for reducing the risk of fuel tank explosions, including reducing the heat input to tanks by ventilation or relocation of the tanks, altering fuel properties such as raising the flash point, and fuel inerting. However, relocation of the fuel tanks is only feasible on new airplanes; full-time ventilation of the fuel tanks is estimated to cost over 3.5 billion dollars over ten years, and alteration of the fuel flash point is estimated to cost over 75 billion dollars worldwide over ten years. Only fuel tank inerting seems to be a viable short term option.
Fuel tank inerting can be effected either on each airplane or on the ground prior to and following flights. In-plane inerting would have the advantage of being tailorable for each plane. Ground inerting would have the advantage of being less expensive to maintain, as it could be a single system at selected airports, rather than a system in each plane. Ground inerting also provides the advantage of not requiring the additional weight on the plane attributable to the presence of an inerting apparatus on the plane. Both methods may be employed together as an extra level of safety that will minimize the amount of time that an explosive mixture exists in the fuel tanks.
Inerting of fuel tanks involves rendering the fuel vapors and other gases (known as ullage) that exist over a supply of fuel less explosive. This generally involves removing the oxygen from the ullage and/or replacing it with an inert gas. Nitrogen is a preferred inert gas because it is relatively plentiful and inexpensive.
Various methods exist for separating mixtures of gases for the production or purification of nitrogen and oxygen. Current methods include both selectively permeable membranes and pressure swing absorption. Selectively permeable membranes work by passing a mixture of gases over a membrane that has a relatively high permeability to one of the gases, and a relatively low permeability to the other gas. For example, a membrane that is more permeable to oxygen than to nitrogen may be used to separate oxygen from an air stream by flowing air through such a membrane. The oxygen will pass through and may be recaptured on the other side of the membrane, while the nitrogen will not pass through. Pressure swing absorption systems use selectively absorbent beds into which a mixture of gases is introduced under high pressure. U.S. Pat. No. 4,194,891 describes such a bed for the separation of oxygen from air. An alternating exposure of high and low pressures causes the beds to release oxygen at a relatively faster rate than the beds release nitrogen.
It is known to try to use inert gases to displace oxygen over supplies of flammable liquids where it is desired to avoid the formation of explosive gases. U.S. Pat. No. 5,858,064 discusses producing nitrogen produced by a gas separation system and a method for introducing nitrogen into and through a crankcase for inerting the interior of the crankcase. But merely blowing an inert gas into a vented fuel compartment is not a desirable or effective solution. A problem that exists is that introduction of a volume of nitrogen into a vented fuel tank will necessarily cause venting of an equivalent volume of ullage from the compartment. This exhaust includes a potentially flammable mixture of oxygen and fuel vapors. Even if this mixture is captured and safely handled, the removal of the displaced fuel vapors can add up to a large loss of valuable fuel and have a significant impact on the environment by the needless dumping of the hydrocarbon fuel into the atmosphere.
A need therefore exists for a relatively inexpensive method and apparatus for inerting the ullage in a fuel tank without losing fuel or fuel vapors while minimizing the risk of fire or explosion and without making the cost prohibitive in terms of equipment or the need for added weight on an aircraft.
The present invention fulfills the above need and fills the voids in the prior art by providing a system for inerting a vented compartment that comprises a system for separating the oxygen from the mixture of air and flammable vapor present in tank ullage and returning the oxygen depleted hydrocarbon/nitrogen mixture to the compartment.
In a first aspect, the present invention relates to a vented controlled atmosphere container inerting system, for inerting a gas comprising a flammable vapor and oxygen, comprising:
(i) a container;
(ii) an outlet in said container for said flammable gas;
(iii) a compressor, in fluid communication with said outlet, which compresses the flammable gas from the container;
(iv) a separator, in fluid communication with said compressor, for removing oxygen from said flammable gas to produce an oxygen-depleted flammable gas;
(v) an inlet in said container, which is in fluid communication with said separator, through which said oxygen-depleted flammable gas is fed into said container.
In a preferred embodiment of this first aspect, the system further includes a supply of make up gas to replace the volume of oxygen removed from the vented container. Preferably, the make up gas is provided by using a second separator for removing the oxygen from a supply of make up gas to replace the volume of oxygen removed from the vented compartment. The system is typically employed with the ullage associated with a fuel tank.
In a second aspect, the present invention relates to a method of inerting a flammable gas, comprising a flammable vapor and oxygen, in a vented controlled atmosphere container comprising the steps of:
(i) removing said flammable gas from said container;
(ii) compressing said gas;
(iii) removing the oxygen from said flammable gas to produce an oxygen-depleted gas;
(iv) feeding said oxygen-depleted gas into said container.
In a preferred embodiment of the second aspect, the method further comprises the step of providing an amount of make up gas to the container to replace the volume of oxygen depleted from the flammable gas. Typically, the flammable gas removed from the container is the fuel ullage associated with a fuel tank.
A preferred embodiment of the second aspect of the invention relates to a method of inerting flammable gas, further comprising the step of feeding a make-up gas into the container to replace the volume of oxygen removed from the flammable gas in the separating step. This is preferably achieved by providing a second separating step wherein oxygen is removed from air and the oxygen-depleted air fed to the container.